A technique in which optical signals of main signal light and dummy light are monitored by using an optical channel monitor (OCM) on a transmitting side and a receiving side of submarine line terminal equipment (SLTE) to monitor a failure in a transmission line has been used. Hereinafter, the optical channel monitor is referred to as an OCM and the submarine line terminal equipment is referred to as an SLTE.
As an example of the optical transmission system described in the background art, FIGS. 7 and 8 each show an example of monitoring of an optical signal for an SLTE by using an OCM in a submarine cable system.
FIG. 7 shows a configuration in which an OCM is disposed on a transmitting side of an SLTE and monitors an optical signal. An SLTE 100A in an A-station will be described. On the transmitting side of the SLTE, a MUX 101A multiplexes on optical signals of a transponder TPD-a being main signal light, and dummy light. An output from the MUX 101A is characterized by an optical spectrum as shown in (a) of FIG. 9. An optical wavelength multiplexed signal from the MUX 101A is branched into a main signal path toward a transmission line leading to a B-station, and an optical monitor path toward an OCM 102A. Like in a measurement principle of a general optical spectrum analyzer, the OCM 102A is capable of measuring an optical power by performing an electrical conversion by using a photodetector while causing an optical bandpass filter to sweep within a narrow wavelength slot. In the monitoring of an optical signal by using the OCM 102A described in the background art, like in a WDM measurement mode for the optical spectrum analyzer, peaks and valleys of the optical signal are discriminated by internal calculation processing, and a peak wavelength (a wavelength at which a peak power is detected) and a peak power (a highest power of the optical signal having peaks and valleys) are automatically specified.
When the OCM 102A detects a peak power, it is determined that there is an optical signal. Further, when the OCM 102A detects no peak power, it is determined that there is no optical signal. Information about the presence or absence of the optical signal is transmitted to a monitoring system 110 in the A-station, whereby monitoring of the optical signal on the transmitting side of the SLTE in the A-station is carried out.
On a receiving side of an SLTE, the optical wavelength multiplexed signal input from the transmission line is separated into individual wavelengths by a DEMUX 103B and the optical signal is then received by a transponder TPD-a. Further, a SLTE 100B in the B-station has a configuration similar to that in the A-station. Information about presence or absence of the optical signal measured by an OCM 102B is transmitted to the monitoring system 110 in the A-station through an out-of-band data communication network (DCN).
FIG. 8 shows a configuration in which an OCM is disposed on a receiving side of an SLTE and monitors an optical signal. A transmitting side of an SLTE is similar to that shown in FIG. 7, except for the optical monitor path for the OCM, and thus description thereof is omitted. An SLTE 200B in a B-station will be described. On the receiving side of the SLTE, an optical wavelength multiplexed signal input from a transmission line is branched into a main signal path toward a transponder TPD-a, and an optical monitor path toward an OCM 202B. An optical spectrum on the receiving side of the SLTE is characterized by an optical spectrum as shown in (b) of FIG. 9. An optical signal in the main signal path is separated into individual wavelengths by a DEMUX 203B, and is then received by the transponder TPD-a. An optical signal toward the OCM 202B is specified by monitoring of the optical signal in the OCM 202B described above.
Information about presence or absence of the optical signal is transmitted to the monitoring system 210 in the A-station, and monitoring of the optical signal on the receiving side of the SLTE in the A-station is carried out. Further, the SLTE in the B-station has a configuration similar to that in the A-station, and information about presence or absence of the transponder TPD-a and dummy light measured by the OCM 202B is transmitted to the monitoring system 210 through an out-of-band data communication network (DCN).
Patent Literature 1 (PTL1) relates to an optical transmission system that multiplexes on a plurality of optical signals and transmits and receives the optical signals through an optical fiber cable. PTL1 proposes a technique in which, when the optical signals having respective wavelengths are not output from an optical fiber cable, it is determined that the optical fiber cable is disconnected, and then output of the optical signals is interrupted.